This invention relates to a method of producing magnetic recording media, and more particularly to a method suitable for the deposition of a magnetic material with its direction of easy magnetization oriented in a direction substantially perpendicular to the film plane.
Referring to FIG. 1 which shows a conventional magnetic recording system, a recording medium 1 consists of a base film 2 and a magnetic layer 3 formed thereon and the direction of easy magnetization of the magnetic layer 3 is horizontal with respect to the film plane (as indicated by the arrow-marks). Accordingly, a recording head 4 associated therewith is constructed and disposed in such a manner that, in recording, it applies a magnetic field with the direction of easy magnetization horizontal with respect to the film plane.
Since the direction of residual magnetization is horizontal to the film plane in the above system, the demagnetizing field in the recording medium is increased as the recording signal is of short wavelength, with the result that the reproduction output is considerably reduced.
In contrast, the short wavelength recording characteristics are remarkably improved in a recording system where the residual magnetization is perpendicular with respect to the film plane of the medium. Such a system will hereinafter after be referred to as a perpendicular recording system. Referring to FIG. 2 which illustrates a perpendicular recording system in schematic representation, a recording medium 5 (briefly, a perpendicular recording medium) consists of a base film 2 and a magnetic layer 6 which is a magnetic thin film (briefly, a perpendicular magnetizing film) having a direction of easy magnetization. perpendicular (indicated by arrow-marks) to the film plane. A recording head 7, used in association with said perpendicular recording medium, is a vertical-type head comprising a ferromagnetic thin film 8 and an excitation coil 9, which applies a magnetic field perpendicular to the film plane of the magnetic layer for recording.
In the system described above, where the residual magnetization on the recording is perpendicular to the film plane of the medium, the demagnetizing field of the medium decreases as the signal becomes shorter in wavelength so that a desirable reproduction output is ensured.
The perpendicular magnetizing layer so far known is a metallic thin film composed of cobalt and chromium as predominant components as formed by the sputtering technique, and having a high coercive force of approximately 1 KOe.
The crystal conformation of the sputtered film based on Co and Cr is a hexagonal closest packing lattice structure when the proportion of Cr is not over about 30 weight percent and in this crystal system, the C axes of unit cells can be oriented in a direction perpendicular to the film plane and the saturation magnetization can be decreased until the anisotropic magnetic field in the perpendicular direction becomes greater than the demagnetizing field, thus enabling a perpendicular magnetizing layer to be matelialied. According to the results of deposition trials, the amount of Cr that yields a perpendicular magnetizing Co-Cr film is in the range of about 13 to 28 weight percent.
However, the sputtering process is slow in the rate of formation of a magnetic layer and does not lend itself to the production of perpendicular magnetizing film at high speed and low cost.
On the other hand, since the base film for a magnetic recording medium is usually a plastic film, such as polyethylene terephthalate film, which is only poorly resistant to heat, the formation of the magnetic film thereon must be carried out under temperature conditions which do not thermally degrade, deform or otherwise impair the base film material.
There has thus been demanded a method which permits production of a perpendicular magnetizing film having satisfactory characteristics at a low temperature which would not adversely affect the base material.